     [target-magnitude to-unit substance]))
 (def energy-as (as-f :ft-lb :J 1.35582))
 (def force-as (as-f :lbf :N 4.44822))
+(def length-as (as-f :m :ft 3.28084))
 (def power-as (as-f :hp :W 745.69987158227022))
 (def pressure-as (as-f :psi :kPa 6.894757293168361))
 (def proppant-concentration-as (as-f :lb-per-gal :kg-per-m3 119.826))

        measurement (generate-measurement-f unit)]
    [measurement (measurement-as-f measurement (first (remove (partial = unit) units)))]))

(generate-measurement-pair (set (quantity-unit-map :density))
                           rand-density-unit typical-density density-as)

(defn rand-measurement
  ([]

((quantity-generate-measurement-map :duration))
((quantity-generate-measurement-map :duration) (rand-unit :duration))
(rand-measurement :duration)
